The present invention relates to a process for producing a contact material for contact pieces of vacuum-contactor tubes essentially consisting of copper and chromium in a proportion of 50 to 70% wt. Cu and 30 to 50% wt. Cr. In addition, the present invention also relates to a contact material produced with the process according to the present invention.
After having gained world-wide acceptance as a leading switch principle for the medium-voltage range, that is approximately in the 7.2 to 40 kV range, the vacuum switch principle is also becoming increasingly significant for contactor applications. Since the development of high-voltage contactors for the voltage range of about 1 to 10 kV, vacuum contactors have also been developed for and applied in the low voltage ranges.
The requirements for vacuum contactors and vacuum medium-voltage circuit-breakers differ fundamentally. Thus, at least 1 million electric operating cycles below nominal current are required of the contactor. Since, in this connection, application cases such as reversing circuit arrangements must be controlled, errors such as electric restrikings, in particular, which can lead to immediate phase-to-phase short circuits, must not be allowed to occur during the breaking operation. In comparison, clearly a lower number of operating cycles is expected of the circuit-breaker, such as 20,000 operating cycles at nominal current. Reversing circuit-arrangements are not customary in the case of circuit-breakers.
As far as the contactor is concerned, currents which are higher by factors than the nominal current must still be able to be reliably switched off and switched on without welding. However, a short-circuit-current breaking capacity is not required as it is of the circuit-breaker, since contactors have line-side fuses.
Corresponding to the diverging requirements for contactor switching properties compared to circuit-breakers, the requirements for the contact material also vary. In vacuum circuit-breakers, contact materials based on CuCr have become generally accepted as the best suited materials. For vacuum contactors, on the other hand, materials such as WCu, MoCu or WCAg--optionally with other additives--are still customary. However, among other things, with a rising number of operating cycles, their switching capacity and dielectric strength deteriorate. In tests to use CuCr materials for contactors as well and to utilize their advantages such as constantly high switching capacity, good gettering capability or high dielectric strength, it turned out that conventionally manufactured CuCr materials--as described for example in the German Published Patent Application 29 14 186, the German Published Patent Application 34 06 535, the German Published Patent Application 25 21 504 or the EP-A-0 178 796--do not fulfill the expectations. With such sintered or sintered impregnation material, problems are caused by the erosion characteristics at a high number of operating cycles. That is, the material loss resulting from erosion exceeds the tolerable limiting values, so that the desired nominal-current number of operating cycles is no longer achieved. Moreover, heavily fissured structures are produced on the switching surfaces. This tends to result in faulty dielectric characteristics in the form of restrikings after current zero.
For the first time, a vacuum contactor with contact pieces of CuCr contact material and a process for producing the contact pieces is specified in the EP-B-0 172 411. According to that process, the contact material is produced by remelting it in the electric arc. The contact material formed possesses a very fine, homogeneous distribution of chromium in the copper matrix and an excellent bonding between both components. Because of these exceptional features, these types of CuCr contact materials exhibit a marked increase in their resistance to erosion. As a result of this increased resistance to erosion, they satisfy the requirements for the vacuum contactor operation and at the same time, the erosion pattern of the contact pieces is uniform thus eliminating the cause of the undesirable restrikings after current zero.
The process of electric-arc remelting can only be applied economically when large-diameter remelting electrodes are used. However, contactors require contact pieces of a relatively small diameter. Thus, the material utilization of the remelted material is comparatively small, which represents a decline in economic efficiency.
Therefore, the object of the present invention is to specify a process with which contact material based on CuCr can likewise be produced for use in vacuum contactors, and to indicate the contact material produced by it.